Cryogenic expansion processes have been well recognized and employed on a large scale for hydrocarbon liquids recovery since the turbo-expander was first introduced to gas processing in the 1960s. It has become the preferred process for high ethane recovery with or without the aid of an external refrigeration depending upon the richness of the gas. In a conventional turbo-expander process, the feed gas at elevated pressure is pre-cooled and partially condensed by heat exchange with other process streams and/or external propane refrigeration. The condensed liquid with less volatile components is then separated and fed to a fractionation column (demethanizer), operated at medium or low pressure, to recover the heavy hydrocarbon constituents desired. The remaining non-condensed vapor portion is subjected to turbo-expansion to a lower pressure, resulting in further cooling and additional liquid condensation. With the expander discharge pressure typically the same as the demethanizer pressure, the resultant two-phase stream is fed to the top section of the demethanizer with the cold liquids acting as the top reflux to enhance recovery of heavier hydrocarbon components. The remaining vapor combines with the column overhead as a residue gas which is then recompressed to pipeline pressure after being heated to recover available refrigeration.
Because the demethanizer operated as described above acts mainly as a stripping column, the expander discharge vapor leaving the column overhead that is not subject to rectification still contains a significant amount of heavy components. These components could be further recovered if they were brought to a lower temperature, or subject to a rectification step. The lower temperature option could be achieved by a higher expansion ratio and/ or a lower column pressure, but the compression horsepower would have to be too high to be economical. Ongoing efforts attempting to achieve a higher liquid recovery have mostly concentrated on the addition of a rectification section and how to effectively increase or provide a colder and leaner reflux stream to the expanded vapor. Many patents exist pertaining to a better and improved design for separating ethane and heavier components from a hydrocarbon-containing feed gas stream.
U.S. Pat. No. 4,140,504 describes methods to improve liquid recovery in a typical cryogenic expansion process by adding a rectification section to the expander discharge vapor, and using the partially condensed liquid as the reflux after it is further cooled and expanded to the top of the rectification section. U.S. Pat. No. 4,251,249 adds a separator at expander discharge, separates liquid from the expanded two phase stream, and sends the liquid to column for further processing. The separated vapor provides refrigeration in a reflux condenser to minimize the loss of heavy components in the overhead vapor stream. In yet another approach, e.g. U.S. Pat. No. 5,566,554, the partially condensed liquid is preheated and expanded to a second separator at an intermediate pressure to yield a vapor stream preferably comprising lighter hydrocarbon components. This leaner stream returns to the demethanizer top as an enhanced reflux after being condensed again and subcooled. The reflux stream so generated is rather limited, and the heavy components not recovered are still substantial.
The most recognized approach for high ethane recovery, perhaps, is the split-vapor process as disclosed in U.S. Patent Nos. 4,157,904 and 4,278,457. In these patents, the non-condensed vapor is split into two portions with the majority one, typically about 65%-70%, passing through a turbo-expander as usual, while the remaining portion being substantially subcooled and introduced to the demethanizer near the top. This higher and colder reflux flow permits an improved ethane recovery at a higher column pressure, thereby reducing recompression horsepower requirements, in spite of less flow being expanded via the turbo-expander. It also provides an advantage in reducing the risk of CO.sub.2 freezing in the demethanizer. The achievable recovery level in these processes, however, is ultimately limited by the composition of the vapor stream used for the top reflux due to equilibrium constraints. Ethane recovery is said to be on the order of 90%, with propane recovery to be about 98%.
The use of a leaner reflux is an attempt to overcome the aforementioned deficiency. One approach is to cool the split vapor stream half way through and expand it to an intermediate pressure, causing partial condensation. The condensed liquid comprising less volatile components is separated in a separator and fed to the demethanizer above the feed from the turbo-expander discharge as the mid-reflux. The leaner vapor so generated is further cooled to substantial condensation and used as top reflux. U.S. Pat. No. 4,519,824 is a typical example. U.S. Patent 5,555,748 further improves this process by cooling the separated liquid prior to entering demethanizer as the mid-reflux. However, the internal pinch expected in the reflux exchanger precludes the capability of generating a higher top reflux flow because it is leaner and at a lower pressure leading to a lower condensation temperature. In addition, the top reflux generated from a single stage separation in the separator utilized is still far from essentially ethane-free.
U.S. Pat. No. 5,953,935 discloses a method to further condition the cooled split vapor by employing a scrub column to produce reflux streams for the demethanizer. The scrub column uses overhead vapor condensate as its reflux stream to produce a bottom liquid stream preferentially containing ethane and less volatile constituents from the two feed streams, the vapor feed at the bottom and the partially condensed feed in the middle. The bottom liquid and a portion of overhead vapor condensate are then flashed to the demethanizer as the reflux to enhance ethane recovery. With the column normally operating at an intermediate pressure, an internal pinch in the reflux condenser, similar to U.S. Pat. No. 4,519,824, often exists and precludes the capability of generating a higher top reflux flow from this scheme. The top reflux flow available for the demethanizer becomes even less when a portion of already limited condensate is required for the scrub column by itself. Although a leaner reflux can be generated for the demethanizer, its advantage is largely off-set by the reduction in its flow rate. In addition, cryogenic pumps and often a reflux drum are also required to facilitate the reflux scheme for the scrub column.
A substantially ethane-free reflux has been introduced in some processes which permits essentially total recovery of ethane and heavier components from a hydrocarbon containing feed stream. These processes recycle a portion of the residue gas stream as the top reflux after being condensed and deeply sub cooled. Because the residue gas contains the least amount of ethane in the entire process, ethane recovery in excess of 98% is economically achievable by providing more and leaner reflux from recycle of a significant amount of residue gas. It should be noted that it is the liquid reflux in contact with, providing refrigeration to, and promoting condensation of the uprising heavy components vapor to enhance liquid recovery. Therefore, the recycle of residue gas must be recompressed to a much higher pressure with penalty on compression horsepower to enable its total condensation.
U.S. Pat. Nos. 4,851,020 and 4,889,545 utilize the cold residue gas from the demethanizer overhead as the recycle stream. This process requires a compressor operating at a cryogenic temperature. Warm residue gas taken from the residue gas compressor, eliminating the need of a dedicated compressor, is disclosed in U.S. Pat. Nos. 4,687,499 and 5,568,737. However, an alternate arrangement with a recycle compressor which is required for a low residue gas pressure scenario and/or permits optimal pressure of recycle residue gas for minor improvement in separation efficiency is also presented in U.S. Pat. No. 5,568,737. Although high liquid recovery is attainable, the system requires increases in capital cost and incurs higher operating costs due to penalty on compression horsepower.
It is desirable for a process to be provided which maximizes ethane recovery but does not require undesirable increases in capital and operating costs.